ijst Theoretical and Experimental Study of Thermal Performance of Flat

International Journal of Science and Technology Volume 5 No. 10, October, 2016

Theoretical and Experimental Study of Thermal Performance of Flat Plate Air Heating Collector K. Agbossou1, F. A. Tetang2, 3, T. E. Boroze1, K. N’wuitcha1, K. Napo1, B. Zeghmati3 1Laboratoire sur Energie Solaire-LES, Université de Lomé, Togo. d’Energétique et de Thermique Appliquée (LETA), Université de N’Gaoundéré, Cameroun. 3Laboratoir de Mathématiques et PhySique-Groupe de Mécanique Energétique- LAMPS-EA4217 ; Université de Perpignan- France 2Laboratoire

ABSTRACT This work presents the survey of the behavior of the constituent of a solar insulator submitted to a natural sunshine in forced convection. A model based on equations of the thermal balances has been established while using the nodal method. Its validation is achieved from the comparison of the theoretical and experimental results. It describes the perfect agreement between the two results. A mathematical model was developed to predict the effect of variation in the input parameters on the collector thermal efficiency. The theoretical results showed that the thermal performance of the collector was sensitive to solar irradiation, ambient temperature, air flow rate, and slat length. The experimental results were in good agreement with the theoretical values. Collector efficiency as high as 62% could be obtained. The results showed that optimal air flow mass (working fluid) ranged from 0.015 to 0.040 kg/s. And the optimum air speed inside the collector was found to be 1.5 to 6 m/s. The measured parameters allowed us to appreciate the theoretical analyses and to valorize our solar air flat-plate for it possible used in solar drier system for agricultural produce drying in Togo. Keywords: Solar Energy, Heat Transfer, Forced Convection, Solar Air Collector, Modeling, Solar Drying.

1. INTRODUCTION Flat plate solar collectors have been in use for very long time. The cooling fluid in such heat exchangers can be air or water; thereby the flat plate solar collector can be divided into two groups known as solar air collector or solar water collector [1, 2]. The solar air collectors are very important in the transformation of solar radiation they receive into usable heat energy. This energy is used in various solar applications, for example, drying of agricultural products, building materials, space heating.

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International Journal of Science and Technology (IJST) – Volume 5 No. 10, October, 2016


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Several types of air dryer has been built and tested worldwide, Sham et al. Ravita et al.[4,5]. The main objective is to collect the maximum amount of solar energy at minimum cost and reduce losses. Many studies have been carried out for this purpose. Many efforts have been devoted by researchers to improve the thermal performance of these solar collectors [3,6]. In other, the work of Oudjedi and al. [7], have shown that the coolant outlet temperature and the absorber are key parameters whose influence the efficiency of solar air collector for drying products. Mokhtar et al., [32] had the same observations in their study. Bahrehmand et al. [30] built a mathematical model for simulating the thermal behavior of single and two glass cover SAC (Solar Air Collector) systems with forced convection flow. Numerous researches have been carried out on PV integrated solar air heating systems [27, 28]. However, air heating collector has been developed by ref [29]. This work focuses on the solar air collector with a transpired cover. The parameters that influence the operation of such a system have been studied. It is then important to make a series of experiments to analyze these parameters and variables that influence this system.

But the tests can be proved very costly (flight test, very expensive instrumentation). This led us to a numerical study and experimental results are compared to results obtained numerically in order to validate our model.

2. MATERIALS AND METHODS 2.1. Numerical study 2.1.1. Description of collector The air flat plate solar collector studied is a simple passes insulator air for heating the drying air flowing between the absorber and the plastic film (UV) in forced convection. The fig. 1 shows the schematic diagram of the collector cover and absorber of this insulator. It consists of a black-painted aluminum plate (absorber), the absorption coefficient αabs = 0.9, isolated from the outside by a polystyrene plate. It is covered with plastic film in polyethylene. The exposure unit has a length of 10.00 m and a width of 2m. The distance between the absorber and the plastic film is 0.02 m (slot height) and the insulation thickness is 0.05 m.

Fig. 1: schematic diagram of the collector cover and absorber

2.1.2. Mathematical modeling In mathematical modeling of solar air collector flat-plate, some assumptions are made to simplify the analysis. These assumptions are listed below:  

reflection is neglected radiation fluxes at the surface of the plastic film and the absorber; space between the insulation and the absorber is negligible;

  

heat transfer and mass-dimensional; the physical properties of materials and constant air; the drying air is considered a perfect gas.

The fig. 2 illustrates the heat transfer processes involved in a tube of an all-glass evacuated solar collector tube with coaxial fluid conduit by using a series of thermal network. It shows how heat is transferred between ambient and working fluid.


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2.1.2.2. The Heat transfer coefficients correlation The convective coefficient taking into account the effect of wind on the outer wall is given by [3] in the form of equation (6) ℎ𝑐𝑣1 = 5,67 + 3,86𝑉𝑤

(6)

Where 𝑉𝑤 the wind velocity (m.s-1), wind density is written by equation (7) 𝜌 = 𝜌0 . 273/(𝑇 + 273)

(7)

With 𝜌0 density at temperature T= 0°C. The convection heat transfer coefficients between the working fluid inside the annulus formed by the delivery plate and the absorber plate and the outside surface of the delivery plate and absorber plate are calculated by Kays et al. (2012) from equation (8). ℎ𝑐𝑣2 = ℎ𝑐𝑣3 =

Fig. 2: System configuration for air heating and its thermal network

2.1.2.1. Energy balance of solar insulator Given the simplifying assumptions made in 2.2.2.1, the equations governing transfers in this solar insulator are given by the equations 1 to 4. Energy balance for the plastic film gives M1 CP1

∂Tv ∂t

For turbulent flow (Re > 7000) in the annulus and delivery tube, the Nusselt number is calculated by the Dittus–Boelter equation (Kalogirou, 2013; Incropera, 2011) [13, 14]. 𝑁𝑢 = 𝑂, 𝑂36𝑅𝑒0,75 𝑃𝑟0,33

∂TFc

hcv3 S2 (Tabs − TFc )

∂t

𝑅𝑒 =

(1)

+ U ∗. ∂T∂xFc) = hcv2 S1 (Tv − TFc ) +

∂Tabs ∂t

𝑎

(11)

(2) ℎ𝑐𝑑 =

= Pabs +hr3 S3 (Tv − Tabs ) + hcv3 S3 (TFc −

Tabs ) + hcd S3

(3)

∂Tiso ∂t

(12)

ℎ𝑟1 = 𝜎 ∗ 𝜀𝐹𝑒𝑥𝑡 ∗ (𝑇𝑐 + 𝑇𝑣 ) ∗ (𝑇𝑐 2 + 𝑇𝑣 2 ) ∗

= hcv4 S4 (Tam − Tiso ) +

hcd S4 (Tabs − Tiso ) + hr4 S4 (Tabs − Tiso )

𝜆𝑖 𝐸𝑖

Radiation heat transfer coefficients between the plastic film cover and the sky can be calculated by the correlations below (Eberlein, 1976) [21],

Energy balance for insulation gives M4 CP4

𝐷ℎ ∗𝑉𝑎

Where 𝑉𝑎 air is speed in the insulator and 𝑎 is kinematic viscosity of air. The balance thermal coefficients by conduction throw collector to insulation is can be calculated by the correlations below (11).

Energy balance for Absorber plate gives M3 CP3

(10)

Where Re is the Reynolds number which is given by:

Energy balance for the working fluid inside the delivery insulator M2 CP2 ∗ (

(8)

𝐷𝐻

Where 𝑁𝑢 is Nusselt number The Nusselt number in the delivery plate surface for laminar flow (500